High electron mobility transistor

ABSTRACT

A high electron mobility transistor comprises a GaN-based electron accumulation layer formed on a substrate, an electron supply layer formed on the electron accumulation layer, a source electrode and a drain electrode formed on the electron supply layer and spaced from each other, a gate electrode formed on the electron supply layer between the source and drain electrodes, and a hole absorption electrode formed on the electron accumulation layer so as to be substantially spaced from the electron supply layer. Since the hole absorption electrode is formed on the electron absorption layer in order to prevent holes generated by impact ionization from being accumulated on the electron accumulation layer, a kink phenomenon is prevented. Good drain-current/voltage characteristics are therefore obtained. A high power/high electron mobility transistor is provided with a high power-added efficiency and good linearity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2000-094574, filed Mar. 30,2000, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a high electron mobility transistor(HEMT), more particularly to a GaN-based HEMT.

It is strongly expected that a nitrogen-compound field-effect transistorusing GaN serves as a power element to be operated at a high power andat a high frequency. The nitrogen-compound field-effect transistorswhich have been proposed are a Schottky gate field-effect transistor,MESFET (metal semiconductor field-effect transistor), HEMT or MODFET(modulated doped field-effect transistor), and MISFET (metal insulatorsemiconductor field-effect transistor). Of them, a GaN-based HEMTemploying Al_(x)Ga_((1-X))N as an electron supply layer is considered asa promising high power element since an electron concentration can berendered higher than that of the GaAs-based HEMT. However, aconventional GaN-based HEMT has a problem in that a kink phenomenonsometimes occurs in the drain-current/voltage characteristics. If thekink phenomenon occurs, a power-added efficiency decreases in a largesignal operation performed at a high frequency. The power-addedefficiency η is defined as η=(Pout−Pin)/VdId, wherein Pout is an outputpower, Pin is an input power, Vd is a supply voltage and Id is a draincurrent. In addition, the distortion increases and the linearitydeteriorates.

Now, the reason why the kink phenomenon occurs in the GaN-based HEMTwill be explained. FIG. 1 is a schematic cross-sectional view of theGaN-based HEMT according to a first conventional example. In FIG. 1,reference numerals 11, 12, 13, 14, and 15 denote a GaN electronaccumulation layer, Al_(x)Ga_((1-x))N spacer layer, n-typeAl_(x)Ga_((1-x))N electron supply layer, Al_(x)Ga_((1-x))N cap layer,and a sapphire substrate, respectively. Furthermore, a gate electrode 16is formed on the cap layer 14, while a source electrode 17 and a drainelectrode 18 are formed on the electron supply layer 13.

In the GaN-based HEMT according to the first conventional example, whena drain voltage increases to raise the intensity of the electric fieldwithin the electron accumulation layer 11, a current of electrons flowsthrough a strong electric field region between the source electrode 17and the drain electrode 18. As a result, pairs 22 of electrons and holesare generated by impact ionization within the electron accumulationlayer 11. The electrons thus generated flow into the drain electrode 18,increasing the drain current a little. However, the effect of theincreased drain current is small. On the other hand, the generated holes23 are accumulated in a lower portion of the electron accumulating layer11 as shown in the figure, due to the absence of the electrode forabsorbing the holes. The potential of the electron accumulation layertherefore decreases, with the result that the drain currentsubstantially increases in a drain-current saturation region of a graphshowing the drain current/voltage characteristics. The drain currentsignificantly increased in this way causes the kink phenomenon shown inFIG. 2.

FIG. 3 is a schematic cross-sectional view of a GaAs-based HEMTaccording to a second conventional example.

Reference numerals 11′, 12′, 13′, 14′, and 15′ of FIG. 3 are a GaAselectron accumulation layer, Al_(x)Ga_((1-X))As spacer layer, n-typeAl_(x)Ga_((1-x))As electron supply layer, Al_(x)Ga_((1-x))As cap layer,and GaAs substrate, respectively. Furthermore, a gate electrode 16′ isformed on the cap layer 14′, while a source electrode 17′ and a drainelectrode 18′ are formed on the electron supply layer 13′.

In the GaAs-based HEMT according to the second conventional examplepairs 22 of electrons and holes are also generated in the electronaccumulation layer 11′ by the impact ionization as described in thefirst conventional example. However, most of the holes are absorbed bythe gate electrode as shown in FIG. 3. Therefore, the holes are notaccumulated in the electron accumulation layer 11′. As a result, thekink phenomenon, a problem of the GaN-based HEMT of the firstconventional example, does not occur in the GaAs-based HEMT in thesecond conventional example.

The big difference of the GaN-based HEMT of the first conventionalexample from the GaAs-based HEMT of the second conventional exampleresides in that a large amount of piezoelectric polarization charges 21are generated in a hetero-junction interface in the former GaN-basedHEMT. This is because the ratio between GaN and Al_(x)Ga_((1-x))N inlattice constant is larger than that between GaAs and Al_(x)Ga_((1-x))Asby an order of magnitude.

When the hetero junction of the GaN layer and the AlGaN layer is formed,positive charges are accumulated in the AlGaN layer near the interfaceat a GaN-layer side, whereas negative charges are accumulated in theAlGaN layer near the interface at a gate-electrode side due to thepiezoelectric polarization effect. As a result, most of the holesgenerated by the impact ionization are prevented from flowing into thegate electrode by the piezoelectric polarization charges (positivecharges) accumulated in the AlGaN layer near the interface at the GaNlayer side. The holes are therefore accumulated in the GaN electronaccumulation layer, causing the kink phenomenon.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide acompound-semiconductor-based high electron mobility transistor whilepreventing a kink phenomenon.

To attain the aforementioned object, the first aspect of the presentinvention provides a high electron mobility transistor comprising:

a GaN-based electron accumulation layer formed on a substrate;

an electron supply layer formed on the electron accumulation layer;

a source electrode and a drain electrode formed on the electron supplylayer and spaced from each other;

a gate electrode formed on the electron supply layer between the sourceand the drain electrode; and

a hole absorption electrode formed on the electron accumulation layer soas to be substantially spaced from the electron supply layer.

According to a second aspect of the present invention, there is provideda high electron mobility transistor comprising:

an electron accumulation layer formed on a substrate;

an electron supply layer formed on the electron accumulation layer, forgenerating a piezoelectric polarization charge of 1×10⁻⁷ C/cm² or morebetween the electron accumulating layer and the electron supply layer;

a source electrode and a drain electrode formed on the electron supplylayer and spaced from each other;

a gate electrode formed on the electron supply layer between the sourceand the drain electrode; and

a hole absorption electrode formed on the electron accumulation layer soas to be substantially spaced from the electron supply layer.

In the high electron mobility transistor, the hole absorption electrode,which is substantially isolated from the electron supply layer, may beformed spaced apart from the electron supply layer in such a manner thatthe hole absorption electrode is not electrically affected by theelectron supply layer. However, it is preferable that the holeabsorption electrode is completely isolated from the electron supplylayer.

The high electron mobility transistor is preferably constituted asfollows.

(1) The hole absorption electrode is formed on the electron accumulationlayer via a semiconductor layer having a smaller bandgap width than thatof the electron accumulation layer.

(2) The hole absorption electrode is formed on the electron accumulationlayer via a p-type semiconductor layer.

(3) The hole absorption electrode is formed of the same material as thegate electrode.

(4) The source electrode is formed between the hole absorption electrodeand the gate electrode.

(5) The hole absorption electrode is formed in parallel with the gateelectrode in a gate width direction and has substantially the samelength as that of the source electrode in the gate width direction.

According to a third aspect of the present invention, there is provideda method of manufacturing a high electron mobility transistor,comprising

a first step of laminating an electron accumulation layer and anelectron supply layer successively on a substrate;

a second step of selectively removing the electron supply layer toisolate an element region;

a third step of forming a source and a drain electrode on the electronsupply layer of the isolated element region; and

a fourth step of forming a hole absorption electrode on the elementaccumulation layer exposed by the selective removal of the electronsupply layer, and simultaneously forming a gate electrode on theelectron supply layer of the isolated element region.

According to the present invention, since the hole absorption electrodeis formed on the electron accumulation layer in order to prevent holesgenerated by impact ionization from being accumulated on the electronaccumulation layer, a kink phenomenon can be prevented. As a result, agood drain current/voltage characteristics can be obtained. It istherefore possible to obtain a high power/high electron mobilitytransistor having a good linearity and a high power-added efficiency.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic cross-sectional view of a GaN-based high electronmobility transistor according to a first conventional example;

FIG. 2 is a graph showing typical drain-voltage/current characteristicsof a high electron mobility transistor having a kink phenomenon causedtherein;

FIG. 3 is a schematic cross-sectional view of a GaAs-based high electronmobility transistor according to a second conventional example;

FIG. 4 is a schematic cross-sectional view of a high electron mobilitytransistor according to a first embodiment of the present invention;

FIG. 5 is a schematic cross-sectional view of a high electron mobilitytransistor according to a second embodiment of the present invention;and

FIG. 6 is a schematic top view of a high electron mobility transistoraccording to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Now, embodiments of the present invention will be explained withreference to the accompanying drawings.

(First Embodiment)

FIG. 4 is a schematic cross-sectional view of a high electron mobilitytransistor (HEMT) according to a first embodiment of the presentinvention. Reference numerals 11, 12, 13, 14, and 15 denote a GaNelectron accumulation layer, Al_(x)Ga_((1-x))N spacer layer, n-typeAl_(x)Ga_((i-x))N electron supply layer, Al_(x)Ga_((1-x))N cap layer,and sapphire substrate, respectively. A gate electrode 16 is formed onthe cap layer 14, while a source electrode 17 and a drain electrode 18are formed on the electron supply layer 13. Furthermore, a holeabsorption electrode 19 is formed for absorbing holes in a recessportion 24. The recess portion 24 is formed for isolation by removing aperipheral portion, other than an element region, of layers 12, 13 and14 to reach the electron accumulation layer 11.

A manufacturing method of the HEMT of the first embodiment is describedbelow. The undoped GaN electron accumulation layer 11 of 2 μm thick isgrown on the (0001) sapphire substrate 15 by a metal organic chemicalvapor deposition (MOCVD method). On the electron accumulation layer 11,an undoped Al_(0.3)Ga_(0.7)N spacer layer 12 of 10 nm, and then, ann-type Al_(0.3)Ga_(0.7)N electron supply layer 13 of 10 nm are formedsuccessively by the same MOCVD method. The electron supply layer 13contains Si as an impurity in a donor concentration of 4×10¹⁸ cm⁻³. Onthe electron supply layer 13, the undoped Al_(0.3)Ga_(0.7)N cap layer 14of 5 nm is formed. Next, a first etching for isolation is performed toform an element region in the form of a mesa, thereby exposing theelectron accumulation layer 11 corresponding to the isolation region 24.Thereafter, a second etching is applied to the cap layer 14 to exposethe portion of the electron supply layer 13 which is to be allowed intocontact with an ohmic electrode to be formed later.

Subsequently, electrode layers 17 and 18 are formed as a source and adrain electrode (ohmic electrodes), by depositing and laminating Ti, Al,Ti and Au in this order from the bottom on the electron supply layer 13exposed by the second etching, followed by subjecting to a heattreatment at 900° C. for 30 seconds. Next, as a Schottky gate electrode,the gate electrode 16 is formed on the cap layer 14 by depositing andlaminating Pt, Ti and Au in this order from the bottom. Furthermore, inthe electron accumulation layer 11 exposed in the previous process, inother words, in the bottom surface of the recess portion 24 formed forisolation, a hole absorption electrode 19 is formed by depositing andlaminating Ni and Au, or Pt, Ti, Pt and Au in this order from thebottom. These materials are selected as electrode materials capable ofohmic contact for holes of the electron accumulation layer 11.

In the first embodiment, the hole absorption electrode 19 is formed tobe in ohmic contact for holes of the GaN electron accumulation layer 11.However, as a large current is not expected to flow for absorbing holes,an electrode material to form a Schottky barrier with the undoped GaNelectron accumulation layer 11 may be selected for the hole absorptionelectrode 19. When the Schottky junction electrode is used as the holeabsorption electrode, the ohmic electrodes for source and drainelectrodes may be formed at first, and, thereafter, the Schottkyelectrode is formed simultaneously with the gate electrode, by adeposition method.

A field-effect transistor having a gate length of 1 μm was formed in thesame construction manner as above. Thereafter, power characteristicswere measured by setting the voltage of the hole absorption electrode 19at the same potential as the source electrode 17 or lower, dependingupon the operation point, in order to absorb the holes. As a result, themaximum value of the power-added efficiency increased by 5% compared tothe conventional structure shown in FIG. 1. In addition, as thethird-order intermodulation distortion was measured at the same outputpower, it decreased by 10 dBc than that of the conventional structure.Therefore, it was confirmed that the power characteristics show goodlinearity.

The reason why the power characteristics are improved is that the kinkphenomenon shown in a drain current/voltage characteristics (explainedin the first conventional example) is suppressed by the presence of thehole absorption electrode 19.

In the device structure according to the first embodiment, holes ofpairs 22 of electrons and holes generated by impact ionization arequickly absorbed by the hole absorption electrode 19 and therefore notaccumulated in the electron accumulation layer. Therefore, the potentialof the electron accumulation layer can be stabilized, suppressing thekink phenomenon. As a result, it is possible to provide ahigh-performance device high in power-added efficiency and low indistortion.

The potential for the holes is lower at a side of the source electrode17. Therefore, the holes generated by impact ionization are accumulatedat the electron accumulation layer near the side of the source electrode17. In this case, if the hole absorption electrode 19 is formed near thesource electrode 17, as shown in FIG. 4, the holes can be efficientlyabsorbed.

In the first embodiment explained above, GaN is used as the electronaccumulation layer 11 and AlGaN is used as the electron supply layer 13.However, the present invention can be effectively applied to anycombination of semiconductor materials employed as the electronaccumulation layer 11 and the electron supply layer 13, as long as apiezoelectric polarization charge 21 of 1×10⁻⁷ C/cm² or more isgenerated by lattice mismatch near the hetero junction interface betweenboth layers 11 and 13. Note that a piezoelectric polarization charge ofabout 4.6×10⁻⁷ C/cm² is generated between Al_(0.1)Ga_(0.9)N and GaNlayers.

(Second Embodiment)

FIG. 5 is a schematic cross-sectional view of a high electron mobilitytransistor according to a second embodiment of the present invention.The feature of the second embodiment resides in that a p-typesemiconductor layer 20 (e.g., p-type GaN layer), or a semiconductorlayer having a smaller bandgap width than that of the electronaccumulation layer 11 is formed on the undoped GaN electron accumulationlayer 11, and thereafter, a hole absorption electrode 19 is formed onthe semiconductor layer 20. In this manner, it is possible to absorbholes more effectively than in the first embodiment. In FIG. 5, likereference numerals are used to designate like structural elementscorresponding to those in FIG. 4 (the first example) and any furtherexplanation is omitted for brevity's sake.

(Third Embodiment)

FIG. 6 is a schematic top view of a high electron mobility transistoraccording to a third embodiment of the present invention. The first HEMTelectrodes are formed of a gate electrode 16-1, a source electrode 17-1,and a drain electrode 18-1. The second HEMT electrodes are formed of agate electrode 16-2, a source electrode 17-2, and a drain electrode18-2. The feature of the third embodiment resides in that the holeabsorption electrode 19 is formed in parallel with the gate electrode16-1 in the gate width direction and has substantially the same lengthas the source electrode 17-1 in the gate width direction. Since thelength of the hole absorption electrode 19 is the same as that of thesource electrode 17-1, the effect of the hole absorption can be madeuniform at any cross section of the drain current direction within theFET, and the kink phenomenon is most effectively suppressed compared tothe case where the hole absorption electrode is shorter than the sourceelectrode. Furthermore, the holes from the two HEMTs can be absorbed bya single hole absorption electrode 19, so that the layout area of theelements can be effectively reduced.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A high electron mobility transistor comprising: an undoped GaN-based electron accumulation layer formed on a substrate; an electron supply layer formed on the electron accumulation layer, a source electrode and a drain electrode formed on the electron supply layer and spaced from each other, a gate electrode formed on the electron supply layer between the source electrode and the drain electrode, and a hole absorption electrode formed on the electron accumulation layer so as to be substantially spaced from the electron supply layer.
 2. The high electron mobility transistor according to claim 1, wherein the hole absorption electrode is formed on the electron accumulation layer via a semiconductor layer having a smaller bandgap width than that of the electron accumulation layer.
 3. The high electron mobility transistor according to claim 1, wherein the hole absorption electrode is formed on the electron accumulation layer via a p-type semiconductor layer.
 4. The high electron mobility transistor according to claim 1, wherein the hole absorption electrode is formed of the same material as used in the gate electrode.
 5. The high electron mobility transistor according to claim 1, wherein a composition of the electron supply layer is AlGaN.
 6. The high electron mobility transistor according to claim 1, wherein the source electrode is formed between the hole absorption electrode and the gate electrode.
 7. The high electron mobility transistor according to claim 1, wherein the hole absorption electrode is formed in parallel with the gate electrode in a gate width direction and having substantially the same length as that of the source electrode in the gate width direction.
 8. A high electron mobility transistor comprising: an undoped electron accumulation layer formed on a substrate; an electron supply layer formed on the electron accumulation layer and generating a piezoelectric polarization charge of 1×10⁻⁷ C/cm² between the electron accumulation layer and the electron supply layer; a source electrode and a drain electrode formed on the electron supply layer and spaced from each other; a gate electrode formed on the electron supply layer between the source electrode and the drain electrode; and a hole absorption electrode formed on the electron accumulation layer so as to be substantially spaced from the electron supply layer.
 9. The high electron mobility transistor according to claim 8, wherein the hole absorption electrode is formed on the electron accumulation layer via a semiconductor layer having a smaller bandgap width than that of the electron accumulation layer.
 10. The high electron mobility transistor according to claim 8, wherein the hole absorption electrode is formed on the electron accumulation layer via a p-type semiconductor layer.
 11. The high electron mobility transistor according to claim 8, wherein the hole absorption electrode is formed of the same material as that of the gate electrode.
 12. The high electron mobility transistor according to claim 8, wherein the source electrode is formed between the hole absorption electrode and the gate electrode.
 13. The high electron mobility transistor according to claim 12, wherein the hole absorption electrode is formed in parallel with the gate electrode in a gate width direction and has substantially the same length as that of the source electrode in the gate width direction.
 14. The high electron mobility transistor according to claim 1, wherein the hole absorption electrode absorbs holes generated in the undoped GaN-based electron accumulation layer.
 15. The high electron mobility transistor according to claim 8, wherein the hole absorption electrode absorbs holes generated in the undoped electron accumulation layer.
 16. The high electron mobility transistor according to claim 1, wherein the hole absorption electrode is negatively biased with respect to the source electrode.
 17. The high electron mobility transistor according to claim 8, wherein the hole absorption electrode is negatively biased with respect to the source electrode. 